Oil recovery process

ABSTRACT

A method of displacing oil from a subsurface, water-sensitive, oil-bearing formation is disclosed. The new method involves determining the aqueous vapor pressure of the water-sensitive formation and injecting into the formation through an input well an oil-continuous displacement fluid having water dispersed therein which fluid has an aqueous vapor pressure no greater than that of the formation, and recovering oil displaced thereby from the formation at a point removed from the point of injection.

Chenevert [54] OIL RECOVERY PROCESS [72] Inventor: Martin E. Chenevert,Houston, Tex.

[7 3] Assignee: Esso Production Research Company [22] Filed: May 4, 1970[211 App]. No.: 34,012 v Related U.S. Application Data [63]Continuation-impart of Ser. No. 726,693, May 6, 1968, abandoned, whichis a continuation-in-part of Ser. Nos. 675,490, Oct. 16, 1967,abandoned, and Ser. No. 699,255, Jan. 19, 1968, abandoned.

[52] US. Cl. ..l66/252, 166/275, 166/305 R, 166/308 [51 Int. Cl. ..E2lb43/22, E2lb 43/25, E21b 43/26 [58] Field of Search 166/252, 268,273-275, 166/305 R, 308; 252/855 D [56] References Cited UNITED STATESPATENTS 3,254,714 6/1966 Gogarty et a1. 166/274 3,123,135 3/1964 Bernardet a].

[4 1 June 20, 1972 3,149,669 9/1964 Binder et a1 1 66/275 X 3,208,5289/1965 Elliott et a1 ..l66/305 R OTHER PUBLICATIONS Moore, John E. Howto Combat Swelling Clays, In Petroleum Engineer, Mar. 1960, pp. 8- 78,-90,- 94,- 95,- 96,- 98 through-101 Primary Examiner-Stephen J. NovosadAttorney-James A. Reilly, John B. Davidson, Lewis l-l. Eatherton, JamesE. Gilchrist, Robert L. Graham and James E. Reed ABSTRACT A method ofdisplacing oil from a subsurface, water-sensitive, oil-bearing formationis disclosed. The new method involves determining the aqueous vaporpressure of the water-sensitive formation and injecting into theformation through an input well an oil-continuous displacement fluidhaving water dispersed therein which fluid has an aqueous vapor pressureno greater than that of the formation, and recovering oil displacedthereby from the formation at a point removed from the point ofinjection.

13 Claims, 8 Drawing Figures PATENTEDJUHZO 1972 3.670.816

sum 1 OF 5 INVENTOR. MART/N E. CHENEVERT ATTORNEY PATENTEflJunzo 1972SHEET 2 OF 5 0 O O I0 O O O (\1 T|Mf I HOURS FIG. 3

j Em INVENTOR. MART/N E. CHENEVERT A TTORNE Y PATENTEDJum I972 SHEET 3OF 5 BALANCED ACTIVITY ACTIVITY OF MUD, o

INVENTOR.

MARTIN E CHENEVERT ATTORNEY WATER CONTENT, WEIGHT "/0 DEPTH FEETPlITENTEflJunzo I972 SHEET I 0F 5 RELATIVE VAPOR PRESSURE, p/p

2 4 6 8 WATER CONTENT, WEIGHT INVENTOR.

MARTIN E. CHENEVERT ATTORNEY on. RECOVERY PROCESS CROSS-REFERENCES TORELATED APPLICATIONS This application is a continuation-in-part ofapplication, Ser. No. 726,693, filed May 6, 1968, now abandoned, whichin turn was a continuation-in-part of application, Ser. No. 675,490,filed Oct. 16, 1967 and application, Ser. No. 699,255, filed Jan. 19,1968, both now abandoned. It is also based in part on pendingapplication, Ser. No. 19,574, a continuation-in-part of application,Ser. No. 726,693, filed Mar. 16, 1970.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention isdirected to methods for displacing oil from a subsurface,water-sensitive, oil-bearing formation with an oil continuousdisplacement fluid having water dispersed therein.

2. Description of the Prior Art Additional recovery techniques involvingthe displacement of crude oil from subsurface earth formations byinjecting a displacing fluid into the formation through an input welland recovering oil displaced from the formation thereby at a pointremoved from the point of injection have been in use by the petroleumindustry for a number of years. These displacement processes normallyinvolve injecting a quantity of a displacing fluid into a permeable,oil-bearing formation at a pressure -sufficient to displace recoverableoil contained within the interstices of the formation. Injectionoperations are normally continued until a substantial percentage of theproduced fluid is comprised of the displacing fluid. The use of thesedisplacement processes as a supplement to or following primary oilrecovery frequently results in substantial increases in the overallrecovery of crude oil from productive formations.

A number of problems are encountered in displacement operations whichare conducted in water-sensitive formations. The nature and extent ofthe difficulties encountered are dependent upon the characteristics ofthe particular reservoir rock to be floodedand are particularly acutewhere the formations contain argillaceous materials. When argillaceousmaterial is contained in the reservoir rock, the introduction of waterinto the formation frequently results in swelling of the argillaceousmaterial which in turn causes. a reduction of matrix permeability. Thispermeability reduction caused by clay swelling may significantly impairthe effective permeability of the reservoir rock, thereby reducinginjectivity and rendering displacement operations much more difiicult.It is therefore desirable that the displacement fluids which contact theclay-containing rock matrix not cause clay swelling with attendantreduction in permeability of the formation.

It is known for instance that employing fresh water as a dis placementfluid where the producing formation contains montmorillonite or otherhydratable clays results in clay swelling. Accordingly, it has beensuggested that water-sensitive formations be flooded with brines or withfluids having oil as the external phase to prevent the argillaceousmaterials from swelling. The oil external fluids frequently includedispersions of water in oil as for example water-in-oil emulsions. Alarge number of additives have been employed in conjunction with theseoil-continuous displacement fluids which contain dispersed water,including the addition of various electrolytes to the aqueous phase fora variety of reasons. The problems caused by swelling of argillaceousmaterials, despite the use of oil continuous fluids have, however,heretofore persisted. There therefore exists a need for a method ofdesigning a displacement fluid that will not result in reduction ofmatrix permeability in water-sensitive formations caused by swelling ofargillaceous materials.

SUMMARY OF THE INVENTION The present invention provides means foralleviating the problems normally encountered when water-sensitiveargillaceous earth formations are contacted with aqueous fluids. Theinvention greatly improves the performance of oil-continuousdisplacement fluids and, while described herein primarily in relation tothe drilling of water-sensitive formations, its applicability todisplacement operations will be apparent to those skilled in the art.

In accordance with the invention, it has now been found that shales,shaley sands, and similar argillaceous formations, in spite of theirextremely low permeability, possess a strong attraction for water andare capable of withdrawing water from water-in-oil emulsions and otherfluids with which they come in contact. This sensitivity to water isevidenced by dimensional changes in response to the absorption ordesorption of water. These changes, although sometimes very slight,contribute materially to formation failure. It has been found that therate at which such a formation withdraws water from a particular aqueousfluid is a quantitive measure of the degree of water sensitivity of theformation in the presence of that fluid. This rate and hence the watersensitivity of the formation can be assessed by at least partiallyimmersing a substantially unaltered sample of the formation in the fluidand measuring the changes in dimensions, weight, or other properties ofthe sample, directly or indirectly, over a selected period. A preferredmethod of measuring the water sensitivity of the formation is to measurethe deformation rate, whether visible or subvisible, of a formationsample in the presence of the fluid.

Although the mechanisms responsible for the transfer of water betweenthe emulsion fluid and the argillaceous shale with which the emulsionfluid comes in contact are evidently complex and are not fullyunderstood, experience has shown that water transfer from the emulsionfluid to the shale will nonnally occur if the vapor pressure of theaqueous phase of the fluid is greater than the vapor pressure of theformation. Measurement of vapor pressures thus provides a convenienttechnique for the evaluation of emulsion fluids. Aqueous vapor pressureis directly proportional to the activity of water and hence watertransfer will normally occur from emulsion to shale when the activity ofthe water contained within the aqueous phase of the emulsion exceedsthat of water contained within the shale. It is important to note thatthe aqueous vapor pressure of the formation normally differs from thevapor pressure of the water or brine contained within the formation. Itappears that certain electrical or absorptive forces associated with thematrix or composition of the formation itself greatly decrease the vaporpressure which the water contained therein would otherwise be expectedto have. Measurement of the aqueous vapor pressure of the formationwhich characterizes the activity of the formation water, is therefore animportant aspect of the invention.

Two general methods for designing oil-base drilling fluids in accordancewith the invention are disclosed. Both involve the addition of vaporpressure depressants to the aqueous phase of the emulsion fluid inamounts sufficient to eliminate or to retard transfer of water from thedrilling fluid to the argillaceous formation. The first method is adirect simulation of the interaction of the fluid and thewater-sensitive formation. A water vapor pressure depressant ispreferably first dissolved in the aqueous phase of the emulsion drillingfluid. The rate of water transfer between this fluid and the formationis then quantitatively determined by immersing a sample of the formationin substantially its natural state in the fluid and determining the rateof deformafion. The concentration of the water vapor depressant can thenbe increased and additional samples tested until a concentration thatreduces the rate of deformation to substantially zero is found. Adeformation rate that for all practical purposes approaches zeroindicates that the fluid can be used with little likelihood of damagingthe formation.

A second method for designing drilling fluids requires that the aqueousvapor pressure for the argillaceous shale formation first be determined.This can be done by exposing formation samples to atmospheres abovedifferent saturated salt solutions having known water vapor pressuresuntil equilibrium is reached. By observing the weight change of thesample resulting from water migration, the vapor pressure of anatmosphere that would result in no weight change is detemrined. Thisvalue represents the formation vapor pressure. After thus determiningthe vapor pressure of the shale formation, an emulsion fluid having anaqueous vapor pressure substantially equal to that of the formation canbe prepared. Such a fluid can be used to drill the water-sensitiveformation with little likelihood of the hole sloughing.

It is still a further aspect of the invention to provide an improvedmethod of displacing oil from water-sensitive earth formations. Theimproved method comprises determining the aqueous vapor pressure of thewater-sensitive formation and injecting into the formation through aninput well an oil-continuous displacing fluid having water dispersedtherein in which the fluid has an aqueous vapor pressure no greater thanthat of the formation, and recovering oil displaced thereby from theformation at a point removed from the point of injection. By maintainingthis relationship between the aqueous vapor pressure of the formationand of the displacement fluid, migration of water from the fluid to theargillaceous formation is prevented, eliminating swelling of theargillaceous material in the matrix and resultant permeabilityreduction. The displacement method of the present invention will thus beseen to have significant advantages over techniques availableheretofore.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically depicts anelevation view of a displacement transducer instrumented with straingauges suitable for performing the simulation test method of theinvention.

FIG. 2 is a schematic plan view of the apparatus of FIG. 1.

FIG. 3 is a schematic diagram of an electrical circuit that can be usedwith the apparatus of FIG. 1.

FIG. 4 graphically illustrates unit elongation versus log time datarecorded while testing a hard shale in accordance with the simulationtest method of the invention.

FIG. 5 graphically illustrates the rate of deformation exhibited by anumber of samples of an argillaceous shale formation contacted bywater-in-oil emulsion drilling fluids having different aqueousactivities.

FIG. 6 shows the water vapor pressure (P), relative to the vaporpressure of pure water (P exhibited by a West Texas hard shale at 25 C.for various water contents within the shale.

FIG. 7 is a correlation showing the average variation in the watercontent of a shale in terms of depth of burial within the earth.

FIG. 8 is a correlation showing the average vapor pressure (P) of twohard shales and one soft shale relative to the vapor pressure of purewater (P,,) at 25 C. for different shale water contents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I. Design of Drilling Fluids A.The Simulation Test Method 1. Nature of the Simulation Test Method Thesimulation test is based on the discovery that the rate at which shalesand other argillaceous formations absorb water from a particular aqueousfluid is a quantitative measure of the degree of water sensitivity ofthe formation in the presence of the fluid. The test is performed by atleast partially immersing a sample of the formation which is insubstantially its natural state of hydration in the drilling fluid ofinterest and determining the rate of water absorption.

One method for determining the water absorbed is by change of weight ofthe sample. The sample is weighed initially and its change in weightobserved over a period of time. Any change in weight which occurs isattributable to the migration of water. Weight measurements can beobtained while the sample is immersed by suspending it in the drillingfluid and periodically recording the suspended weight. In lieu of this,the sample may be withdrawn from the fluid after a fixed period of time,cleaned, and then weighed. Another method recognizes that theresistivity of the sample will decrease as it absorbs water and utilizeschanges in resistivity to measure the amount of water absorbed. Stillother methods are based on the measurement of changes in sonic velocity,compressive strength, and other physical properties which vary withwater content to indicate the rate of absorption.

The preferred method of measuring absorption is to log the rate ofchange in dimensions of a shale sample while it is immersed in thedrilling fluid. This gives a direct measurement of the deformation ofthe shale due to the drilling fluid and thus provides a quantitativemeasurement of the rate of water ab sorption. A wide variety of devicesfor recording changes in dimensions may be used, including micrometers,optical equipment, dial displacement indicators, and the like. Thepreferred apparatus, however, is a displacement transducer instrumentedwith strain gauges.

2. The Displacement Transducer Apparatus FIGS. 1 and 2 illustrate aresistance strain gauge displacement transducer suitable for measuringthe change in dimensions of a sample of shale or similar material. Thisapparatus includes a rectangular base 10 from which a substantiallycylindrical column 12 extends vertically. A series of beveled teeth onthe upper portion of column 12 form rack 14.

Cantilever deflection beam 22 engages rack 14. The outermost end of thedeflection beam extends downwardly in an I..- shape terminating in afrustoconical end terminus 24. Contactor shoe 21 is mounted on endtemiinus 24. The innermost end of the deflection beam 22 contains agenerally oval aperture 23, one end of which forms a yoke that fits overupright column 12 and forms a slidable support with the column. Shaft 18passes through deflection beam 22 at the other end of aperture 23. Knobs16 are mounted on the ends of shaft 18. Pinion 20 is supported on theshaft 18 in a position corresponding to the middle of the aperture tocooperate with rack 14. Upper strain gauges 25 and 26 are mounted on theupper side of deflection beam 22. Lower strain gauges 27 and 28 are positioned on the other side of the beam.

A cylindrical pedestal 30 extends from a rectangular base 10 underneathcontactor shoe 21. The upper surface 31 of the cylindrical pedestal issmooth and forms a bearing surface underneath shale sample 34.Cylindrical cup 29 slides upon cylindrical pedestal 30. Sealing member32 is mounted between the cup and the pedestal to prevent the leakage offluids.

FIG. 3 illustrates an electrical circuit suitable for use with thestrain gauge displacement transducer apparatus. A fourresistorelectrical bridge in which strain gauges 25, 26, 27, and 28 form theresistors is shown. At least four resistors are generally used to obtainincreased amplitude and inherent temperature compensation. Variableresistor 32 is placed in the circuit to balance the bridge prior tostrain measurements. Voltage source 35 creates a difference in potentialacross resistor 32 and across the bridge between contacts 36 and 37,causing direct current to flow through resistor 32 and the legs of thebridge formed by resistors 25 and 27, an 26 and 28, respectively.Voltage is measured between terminals 40 and 42 by voltmeter 44. In lieuof this, a suitable strain indicator, such as Model P-350 sold by TheBudd Company, Phoenixville, Pennsylvania, could be used. Switch 46 isused to turn the strain gauge transducer on and off. Although therelatively simple strain gauge circuit illustrated is suitable, othercircuits such as those illustrated in M. Hetenyis book, Handbook ofExperimental Stress Analysis, John Wiley & Sons, Inc., New York, NewYork 1950) could readily be adopted.

Prior to using the strain gauge transducer, it must be calibrated todetermine the relationship between observed voltages and displacement.This can be done by first zeroing the voltmeter, as is discussed below,and then placing successively larger or smaller articles of known lengthbetween contactor shoe 21 and cylindrical pedestal 30 and observing thevoltages. From these data a constant that relates voltage anddisplacement can be obtained.

, To use this equipment to analyze the compatibility of a drilling fluidand a particular shale, a sample of the shale should be placed onsurface 31 of cylindrical pedestal 30. Deflection beam 22 is thenlowered by turning know 16. This rotates shaft 18 on which pinion 20 ismounted. Pinion 20 cooperates with rack 14 to convert the rotationalmovement of the knob 16 into a downward translational movement of beam22. The beam should be lowered until contactor shoe 21 engages the shalesample 34 and holds it firmly in place on surface 21 of the pedestal 30.

With the shale sample thus mounted, the strain gauge's electricalcircuit should be balanced. Voltage source 35 is energized by closingswitch 46, causing current to flow through variable resistor 32 and bothsides of the resistance bridge. The bridge is balanced by adjustingvariable resistor 32 until voltmeter 44 is zeroed. Once the bridge hasbeen balanced, the voltage readings will indicate deformation.Cylindrical cup 29 is then raised to its uppermost position so that theupper edges of the cup are above the top of sample 34. Sufficientdrilling fluid to cover the sample is then poured into the cup heldbetween contactor shoe 21 and surface 31 of cylindrical pedestal 30.

Once the drilling fluid contacts the sample mounted within the straingauges, the sample will begin to absorb water and expand if it isincompatible with the fluid. Expansion of the sample will forcecontactor shoe 21 upward, deflecting beam 22. Deflection of the beamresults in deformation of the strain gauges and produces an imbalance involtage readings across the bridge. If the fluid absorbs water from thesample, the sample will generally exhibit shrinkage. Such shrinkage alsonormally produces an imbalance in voltage readings across the bridge.However, these voltages will have an opposite sign from those caused byswelling.

Several voltage readings should be taken at various times after thesample has been immersed in the drilling fluid. The voltage readings areproportional to the displacement of the sample between contactor shoe 21and pedestal 30. The relationship between displacement and time can bedetermined from the calibration constant and used to determine the rateat which this sample will absorb water from the particular drillingfluid. When comparing data, it isuseful to normalize the displacementdata by dividing each reading by the sample length. The normalized datais then referred to as strain." The rate so determined is indicative ofthe degree of compatibility between the water-sensitive formation andthe drilling fluid.

3. Selection and Preparation of Formation Samples The determination ofthe water sensitivity of a subsurface formation in the presence of aparticular drilling fluid in accordance with the invention is normallycarried out with a sample of the formation having substantially its insitu composition. Exposure to high temperatures and other treatment thatmay alter the composition should be avoided. It is preferred that thissample be in substantially its natural state of hydration so that itssurface absorption behavior will approximate in situ absorptionbehavior. Laboratory tests performed at reservoir conditions oftemperature and pressure, when compared with absorption tests conductedunder atmospheric conditions, indicate that atmospheric tests aresufficiently accurate for most practical purposes.

The formation samples utilized may be preserved core samples from thesubject well or from a nearby well that penetrates the same formation.Such preserved samples are particularly representative when the coringfluid used inhibits absorption of water by the water-sensitiveformation. Fragments of the formation entrained by the drilling fluidand carried to the surface can also be used. Since a water-sensitiveformation will begin hydration as soon as it is contacted with awater-containing drilling fluid, it is preferable that such fragments berecovered as early as possible after initial contact of the rock by thefluid. Hence, the depth of the formation of interest should be estimatedand samples from the earliest returns from drilling the formation shouldbe secured for the test. The use of an oil-base drilling fluid treatedin accordance with the invention generally simplifies the recovery ofsamples in substantially their natural state of hydration.

Where severe hydration of the formation has occurred, the samplesobtained should be restored to their natural state of hydration.Hydration is not always encountered when the drilling fluid is a treatedoil-base fluid and is generally more severe where a water-base fluid isused to drill a highly watersensitive shale. Restoration to asubstantially natural state can be accomplished by baking the samples ata temperature slightly above I00 C. until sample density correspondswith typical shale density for this formation and depth of burial.Sample density can be rapidly determined by means of a graduated densityliquid column, the mercury pump pressure chamber method, or othersuitable techniques. Correlations of shale density versus depth ofburial are available in the literature for various formations and aretypified by those published by K. F. Dallmus in his study "Mechanics ofBasin Evolution and Its Relation to the Habitat of Oil in the Basin,Habitat of Oil A Symposium, Tulsa, Amer. Assoc. Petrol. Geol., 1958, p.883-931. lt is important that temperature not greatly exceed C. sinceexcessive temperatures may result in substantial changes incharacteristics of the sample.

4. Drilling Fluid Design Use of the method and apparatus of theinvention to formulate an oil-base drilling fluid that will prevent orminimize ab sorption and thus promote borehole stability is based inpart on the observation that an oil-base or water-in-oil emulsion mudhaving an aqueous vapor pressure substantially equal to or less thanthat of the troublesome water-sensitive fonnation will preventabsorption of water by the formation. Samples of the water-sensitiveformation in substantially their natural state should be used, asindicated above. Several of these samples are preferably immersed in acorresponding number of diflerent oil-base drilling fluids havingdifferent aqueous vapor pressures and strain-time data are obtained foreach formation combination, This procedure can be greatly expedited byusing a number of strain gauge displacement transducers.

A series of water-in-oil emulsions or other oil-base muds havingdifferent aqueous vapor pressures can be prepared by adding variousconcentrations of inorganic salts such as NaCl or CaCl, to the mud. Anumber of other vapor pressure depressants are discussed herein inconnection with the method of determining the vapor pressure of an earthformation. Suitable vapor pressure depressants are not limited to theseor similar inorganic salts, however. Any solute introduced into theaqueous phase will reduce the aqueous vapor pressure.

FIG. 4 illustrates strain-time date obtained in accordance with theinvention for the hard, argillaceous Wolfcamp shale. Fluid A is water,and the high rate of absorption for this fluid is typical of a verycompatible fluid. Fluids B, C, D, and E are water-in-oil invertemulsions containing in the aqueous phase, as vapor pressuredepressants, 130,000-ppm NaCl, 200,000- ppm NaCl, 270,000-ppm NaCl, and450,000-ppm CaCl,, respectively. Curves B, C, and D illustrate thereduction in absorption that occurs as the concentration of theaqueousvapor pressure depressant is increased and the aqueous vapor pressure ofthe fluid approaches that of the formation. Curve E illustrates behaviorcharacteristic of a water-in-oil emulsion mud with an aqueous vaporpressure that has been reduced below that of the water-sensitiveformation. Instead of swelling, the shale sample shrinks, indicatingthat water is being desorbed from the shale sample. The use of adrilling fluid with a composition similar to that of mud E wouldtherefore prevent absorption of water by the shale. Generally, however,there is little incentive in attempting to dehydrate a water-sensitiveformation and therefore such a fluid would normally be considered tocontain an excessive amount of vapor pressure depressant. In most casesit would be more economical to reduce the concentration of CaCl, inFluid E so that its strain-log-time curve would more closely approachthe zero strain line than to use a mud such as Fluid E.

P16. illustrates graphically the rates of deformation of a series ofshale samples exposed to invert muds having varying aqueous activities(relative vapor pressures). The shale formation on which the tests wererun had an aqueous activity of 0.7. Each test involved immersing a shalesample in an invert mud having a known aqueous activity for a period ofhours, measuring the strain, and then computing the average rate ofstrain of the sample over this time period. It will be noted that shalesamples exposed to muds having aqueous activities higher than 0.7swelled and that the observed rate of swelling increased as thedifference in aqueous activity between the mud and the sample increased.Samples contacted with muds having aqueous activities lower than 0.7shrank. Again, however, the rate of deformation increased in relation tothe activity difference. These data demonstrate that when a differencein activity exists, water will flow either from the emulsion mud to theshale or from the shale to the mud. The former causes swelling of thewater-sensitive subterranean formation, leading to its sloughing intothe wellbore; the latter increases the water content and thus viscosityof the drilling fluid, necessitating frequent additions of oil, salt,and other materials required to maintain the drilling fluid. However,when the activity of the mud is substantially equal to that of the shaleformation being drilled, a unique relationship exists. So long as thisbalanced condition is maintained, there is substantially no migration ofwater in either direction. Thus, it is especially desirable to maintainthe aqueous activity of the mud about equal to that of the shale andthereby both eliminate sloughing of the borehole and obviate theaddition of salt, oil or other materials to the mud normally requiredwhen it is contaminated by water.

Although the simulation test has been discussed in relation towater-in-oil emulsion drilling fluids the utility of the simulation testis not limited to this type of drilling fluid. The simulation testmethod and apparatus can be used to determine the compatibility of anydrilling fluid with a water-sensitive formation and can be employed toselect the most compatible drilling fluid from any group of drillingfluids. The method and apparatus can also be used to determine whetheror not a particular formation is water-sensitive and to select fluidsfor use in secondary recovery, well stimulation, or other welloperations, as is more fully discussed subsequently herein.

B. The Formation Vapor Pressure Test Method 1. The Method of Determiningthe Vapor Pressure of an Earth Formation The aqueous vapor pressure of ashale or other water-containing earth formation can be determined bysubjecting a sample of the formation to air of a constant known humidityfor a period of time sufficient for moisture within the shale to reachequilibrium with the moisture in the air. It will normally be difiicultto preselect a humidity condition such that the natural water content ofthe shale will be in equilibrium with this condition of humidity. So,generally speaking, several different humidity conditions must be usedto obtain a range of water contents within the sample which will spanthe in situ water content of the formation within the earth.

A very convenient procedure for exposing samples of a given formation toair of difierent humidities is to'suspend or place the sample in asealed container in an atmosphere of air above a saturated aqueoussolution of a solute which contains an excess of undissolved solute.Thus, it is known that the relative humidity of the enclosed space abovesuch a soluu'on where the sample has been placed will remainsubstantially constant at a given temperature conveniently roomtemperature (25 C). An article containing an explanation of thisprinciple and also listing a number of saturated solutions and solutesis contained in Ecology: Vol. 41; No. 1; pp. 232-237 Jan., 1960).Typically, a series of several different saturated solutions can beprepared, and one or more samples of a given shale or other formationcan be exposed to an enclosed atmosphere above each of these samples fora suflicient period of time for equilibrium to occur. Completeequilibrium will normally take about 1 or 2 weeks, but substantialequilibrium can normally be attained in about 1 or 2 days.

exist above these solutions are listed below:

Relative humidity Saturated solution of (I) at 25C ZnCl, 1% H O l0.0CaCl, 611,0 29.5 Ca(NO,), 411,0 50.5 NH Cl KNO, 71.2 (N11,), S0 80.0 NaTartrate 92.0 Kl-LPO, 96.0 K Cr,O 98.0

Many of these salts, incidentally, may themselves be used within theaqueous phase of invert emulsions for the purpose of establishing thevapor pressure of that phase. If vapor pressures less than thatobtainable for a saturated calcium chloride solution are desired,solutions of ZnCl, l KILO; LiCl H,O; ZnBr,; LiBr 2H,O; potassiumhydroxide or other stronger vapor pressure depressant may be employed.The depressant, of course, must be compatible with the invert emulsionof interest; and such compatibility should be tested prior to actualuse.

After a formation sample has reached equilibrium with a particularatmosphere of known relative humidity, the sample should be withdrawnfrom the atmosphere and its water content promptly determined. A sirnpleprocedure for determining its water content is to weigh the equilibratedsample, and then repeat the weighing after the sample has been dried atabout C. for a period of 12 to 24 hours. The loss in weight of thesample is a direct measure of the equilibrated water content of thesample. The vapor pressure of the sample for this water content is thevapor pressure of water at room temperature (or the temperature of theequilibrium condition) multiplied by the percent relative humidity ofthe air in equilibrium with the sample. 7

After the vapor pressures and water contents of a given sample or set ofsamples have been determined, these values can be recorded on a suitablechart or other record medium and intennediate values can be determinedfrom the resulting correlation. Thus, FIG. 6 of the drawing shows twocorrelations (A" for absorption conditions, and D for desorptionconditions) obtained by subjecting samples of a West Texas hard shale toeight difierent conditions of relative humidity ranging from 10 percentrelative humidity to 98 percent relative humidity at a temperature of 25C. These curves also apply for temperatures at least as high as l00 C.As can be seen, slightly diflerent correlations were obtained for testsin which water was desorbed from the shale samples as compared withtests in which water was absorbed by the shale samples. The shale samplein this instance had an in situ water content of 2.22 weight percent asdetermined by analyzing a small central portion of a core cut directlyfrom the fonnation under conditions such that the water content of mostof the core was undisturbed. From FIG. 6, it is apparent that thisformation has a water vapor pressure (for formation vapor pressure")relative to the vapor pressure of pure water of between about0.7land0.8l.

Another convenient method for determining the aqueous vapor pressure ofa water-sensitive formation is to place a sample that is representativeof the subsurface formation in a sealed container until it reachesequilibrium with the enclosed atmosphere. A direct measurement of therelative humidity of the formation sample can then be made. This methodis also useful for determining the aqueous vapor pressure of awatercontaining oilabase drilling fluid. Apparatus for measuringrelative humidity is widely available. Typical of such apparatus is theCatalog No. 2200 ELECTRO-HYGROMETER that is sold by Lab-Line instrument,lnc., Melrose Park, I]- linois.

2. Selection and Preparation of Samples As indicated earlier, the use ofthis invention in designing well fluids should be preceded by adetermination of the vapor pressure characteristics of the portions ofthe zones or formations which the emulsion fluid will contact. In thecase of a drilling operation, as pointed out earlier in the discussionof the simulation test, a sample of the formation of interest should beobtained so that its vapor pressure can be determined. if a sample ofthe formation is not obtainable directly from the well being drilled,then an effort should be made to obtain a sample from a nearby well. Itis also possible, however, to collect and use cuttings from the wellwhich is being drilled.

Again, the preferred type of formation sample to obtain and study is asample from the central portion of a core which has been cut from theformation under conditions suitable to preserve the natural conditionsof the core as much as possi ble. if such a sample is available, areasonably accurate determination can be made of the amount of in situwater contained in the core. If such a core cannot be obtained, theformations content can be estimated from FIG. 7. FIG. 7 is a correlationshowing how the water content of many shaley formations within the earthvary, on the average, with increasing depth of burial. Thus, if a givenformation lies about 10,000 feet beneath the surface, it may be expectedto have, on the average, a water content of about 2 weight percent. Itis then possible to use this water content, in combination with themethod described earlier for determining the vapor pressure of aformation within the earth, to arrive at an approximate value of thevapor pressure possessed by the formation in its natural conditionwithin the earth.

3. Drilling Fluid Design Once the formation vapor pressure is known, itis then possible to select and formulate a water-in-oil drilling fluidhaving an aqueous phase vapor pressure which bears a particular relationto the aqueous vapor pressure of the formation. Generally speaking, itis desirable that the aqueous phase of the drilling fluid have anaqueous vapor pressure no greater than that of the water-sensitiveformau'on. This frequently requires the aqueous vapor pressure of thedrilling fluid to be less than that of a saturated sodium chloridesolution and often it is desirable to saturate the aqueous phase of thedrilling fluid with calcium chloride. As pointed out above with respectto drilling fluid design by the simulation method, it is especiallydesirable to maintain the aqueous activity of the mud at a level aboutequal to that of the water-sensitive formation. Balancing the activitiesin this fashion eliminates any substantial migration of water betweenthe emulsion fluid and the formation, thereby eliminating any sloughingof the borehole as well as contamination of the mud by water containedwithin the shale.

However, economics or other considerations may occasionally make itundesirable to attempt to completely reduce the aqueous vapor pressureof the drilling fluid to that of the formation. As long as the watertransfer between the fluid and formation is insufficient to causeexcessive formation failure during the time period the water-sensitiveformation is exposed to the wellbore, the aqueous vapor pressure of thedrilling fluid can be considered to be substantially equal to that ofthe water-sensitive formation. However, it is preferable to reducedrilling fluid vapor pressure to a level that is equal to or below thatof the formation.

It should be noted that mixtures of salts can be used in the I waterphase of an invert, but such mixtures are subject to the common-ioneffect. Their aqueous solutions may thus have higher aqueous vaporpressures than would otherwise be suggested by the total saltconcentration. It should also be noted that the emulsifier and otherwater-soluble constituents of the drilling fluid may tend to slightlyalter the vapor pressure of the aqueous solution containing the vaporpressure depres sants when the emulsion fluid is prepared. Thus theaqueous vapor pressure of the emulsion fluid, which is the aqueous vaporpressure of the water phase of the emulsion fluid, may differ slightlyfrom that of the aqueous salt solution used to prepare the emulsion.Generally speaking, however, an invert emulsion drilling fluid whereinthe aqueous phase is saturated with sodium chloride may be used wherethe vapor pressure of the formation has a value (P) about three-fourthsof the vapor pressure of water (P,) at the same temperature (i.e., arelative vapor pressure of 0.75). Referring to FIG. 8, such a mud wouldbe successful in drilling the deep, hard, West Texas shale (A) shownthere which possesses a natural water content of about 2.2 weightpercent. In that regard, it should be noted that hard argillaceousshales seldom exhibit relative aqueous vapor pressures in excess of0.75. The deep, hard, Louisiana shale (B), for example, would normallybe drilled with an invert emulsion fluid wherein the aqueous phaseconsists of saturated calcium chloride solution (i.e., a relative vaporpressure of 0.30). This shale has a connste or natural water content ofabout 1.8 or 1.9 weight percent. The soft, gumbo shale (C) has a connateor natural water content of about 1 1 weight percent and is bestsatisfied by a fluid with an aqueous phase vapor pressure less than asaturated aqueous NaCl solution.

C. Emulsion Drilling Fluids Designed by the Methods of the Invention.

Referring specifically to water-in-oil invert emulsion drilling fluids,a variety of such fluids are commercially available for use in drillingwells. Fluids of this type may be modified by the addition of vaporpressure depresants and can be used for drilling through water-sensitiveformations difficult to drill with the commercial fluids. Typical invertemulsion drilling fluids contain droplets of water finely dispersed oremulsified in an, oil base. Diesel fuels, kerosenes, and high-gravitycrude oils are frequently used as the oil base; and about 10 to 70percent of fresh or common salt water is emulsified therein with thehelp of suitable emulsifying and stabilizing agents. Anionic, nonionic,and mixed anionic-nonionic emulsifiers are all used for this purpose.The emulsifiers and stabilizing agents employed in the fluids should becompatible with sodium chloride, calcium chloride, or whatever watervapor pressure depressant is to be incorporated in the aqueous phase ofthe modified compositions. One specific invert emulsion drilling fluidcomposition which has been tested and appears satisfactory for manyapplications comprises 70 volume percent No. 2 diesel fuel; 25 volumepercent water saturated with calcium chloride; and 5 volume percentsorbitan monooleate as the emulsifier. No difficulty, however, has beenencountered in obtaining other satisfactory compositions simply byadding sodium chloride or calcium chloride to certain existingcommercially available invert emulsion drilling fluids. Where formationsare particularly water-sensitive it may be desirable to prepare adrilling fluid having a-vapor pressure which is less than that of a mudcontaining a saturated calcium chloride solution. Solutions containingZnBr,, ZnCl,, LiBr, LiCl, or similar water-soluble salts can be employedfor this purpose. ln addition, it has been found that a supersaturatedCaCl, mud can be formed by adding additional CaCl, to a mud having asaturated CaCl, solution as the aqueous phase. Such supersaturated CaCl,muds have vapor pressures lower than those of saturated CaCl, muds.Other water vapor depressants contemplated to be useful in the variousembodiments of this invention include still other water-soluble salts;phosphoric acid, acetic acid, and other water-soluble acids; glycerol;sodium hydroxide; potassium hydroxide; etc.

D. Monitoring the Drilling Fluid at the Wellsite Once a compatibledrilling fluid has been selected and introduced into the drillingsystem, it is advisable to monitor the fluid periodically to insureretention of compatibility. Contaminants, absorption, and otherphenomena may cause gradual changes in the composition of the mud.Monitoring llll can be rapidly accomplished by periodically immersingsamples of successive formations penetrated by the well in portions ofthe mud in contact with these formations and logging the direction andextent of water migration between each such sample and the mud in whichit is immersed with the displacement transducer apparatus.

In some cases it may be desirable to monitor the mud with calibratedshale samples having known vapor pressures. These calibrated samples maybe preserved samples of the formation being drilled that have been takenfrom another well. Synthetic shale specimens, clay specimens, and thelike, prepared so that they have particular aqueous vapor pressures canalso be used. Calibrated shale samples representative of thewater-sensitive formation are equivalent to substantially unalteredformation samples and the necessity for obtaining such samples from thewell being drilled can thus be eliminated. By comparing an oil-basefluid of unknown aqueous vapor pressure with shale samples having knownaqueous vapor pressures, it is apparent that the vapor pressure of thefluid can be determined. In this connection, it may be desirable tocontinuously monitor the aqueous vapor pressure of the oil-base mud withthe displacement transducer and compare it with the formation vaporpressure. Another convenient method to monitor the aqueous vaporpressure of the mud is to place a sample in a closed container anddirectly measure the relative humidity of the atmosphere in contact withthe samples as is discussed above.

The condition and composition of the oil-base fluid can be determined byperiodically emulsion-breaking a mud sample and determining its watercontent. In addition, the water can be analyzed for its content of vaporpressure depressant. Thus, if the vapor pressure of the aqueous phase isbeing controlled by the presence of calcium chloride, the aqueous phasecan be analyzed for this salt.

The condition of the drilling fluid can also be qualitatively evaluatedby observing the cuttings produced in the drilling operation if thewater-sensitive formation is such that it will undergo visibledeformation as it absorbs water. If the cuttings are firm and uniform,it can therefore be inferred that the fluid and the formation are insatisfactory condition. n the other hand, if the cuttings become softeror more diffuse, the concentration of the vapor pressure depressant inthe aqueous phase of the fluid should be increased. 1

If an invert emulsion drilling fluid prepared in accordance with theinvention loses water from its aqueous phase during drilling, it isprobable that the water is being absorbed by the surrounding formation;and if this is the case, drilling conditions will tend to become moreadverse. It is therefore desirable, under such circumstances, to addvapor pressure depressant to the aqueous phase of the fluid until itsaqueous vapor pressure is no greater than the aqueous vapor pressure ofthe formation being drilled. This can generally be done by vigorouslymixing the fluid at the surface of the earth with fresh depressant.

As noted previously, drilling fluids prepared in accordance with thepresent invention are especially applicable for use in the drilling ofhard shales. Until the advent of this invention, there has been nosatisfactory procedure for dealing with such shales. As notedpreviously, such hard shales generally have aqueous activities less than0.75 which corresponds to a saturated solution of NaCl. The resultsobtained in accordance with the invention have shown that invertemulsion muds wherein the aqueous phase is water saturated with calciumchloride are remarkably effective for a wide variety of such shales. If,during the course of drilling such a shale, additional calcium chloridemust be added to the mud system, this may be done by mixing powderedcalcium chloride into the fluid. Powdered calcium chloride has beenfound to readily enter the aqueous phase of an invert emulsion drillingfluid.

Abnormal pressure zones represent serious drilling hazards in many areaswhere wells are drilled. One characteristic of such zones is atransition zone that lies just above the abnorcontent. Since acorresponding increase may also be observed in the water activity ofshales in the transition zone, continuously logging the activity offormations penetrated provides a method of detecting abnormal pressurezones. Water activity of a shale is reflected by the ratio of itsaqueous vapor pressure to the vapor pressure of pure water at the sametemperature, i.e., relative humidity. It may therefore be desirable tolog the aqueous vapor pressure of the drill cuttings of the formationsas they are penetrated. Measurements can be performed by exposing thecuttings to atmospheres of varying known humidities as discussed above,by placing the cuttings in a closed container and directly measuring therelative humidity of the atmosphere in contact with them as alsodiscussed above, or by using the displacement transducer apparatus inconjunction with a series of oil-base fluids having known aqueous vaporpressures. If the aqueous vapor pressure of the shale is equal to thatof the oil-base fluid, when the shale sample is placed in contact withthe fluid it will exhibit no deformation.

ll. Use of the Methods of the Invention for Other Fluids A. TreatingFluids The principles of this invention are also applicable to otherwiseconventional well fluid compositions such as packer fluids, coringfluids, completion fluids, and well treating fluids. With respect totreating fluids, for example, the methods and apparatus of the inventionare useful in designing fluids for repairing and restoring water-damagedformations. In the past, it has been conventional practice in the fieldto attempt to restore water-damaged formations by treating them withconcentrated salt water (30,000-50,000 ppm) or with so]- vents such asalcohols which have at least some degree of miscibility with both waterand hydrocarbons. In accordance with the present invention, a suitabletreating fluid is a water in-oil emulsion wherein the aqueous phase hasa sufficiently low vapor pressure so as to attract water from thedamaged formation, thereby dehydrating and restoring the formation.Suitable oils for use in the emulsion include diesel fuels, kerosenes,light fuel oils, light crude oils, light petroleum fractions, LPGs, andthe like. Oil-base and water-in-oil invert emulsion drilling fluidscontaining vapor pressure depressants are also generally suitable foruse as packer fluids, coring fluids, completion fluids, etc.

B. Displacement Fluids The principles of the invention are alsoapplicable to fluid compositions and methods used in displacing oil fromreservoirs. In recent years, for example, it has been observed thatwater-in-oil emulsions and microemulsions are useful in displacing oilfrom reservoirs. Such fluids generally are prepared from the same typesof oils used to prepare invert emulsion treating fluids. Soluble oilshave been employed to form microemulsion displacement fluids. Suchformulations are typified by the displacement fluids disclosed in US.Pat. No. 3,254,714. The emulsion or microemulsion is injected into areservoir at one point and driven from that point through the reservoirtoward a second point where displaced oil is recovered from thereservoir. Since such emulsions and microemulsions have a substantialdegree of miscibility with reservoir oils, and since their viscositiescan be controlled to a considerable degree, they appear attractive foruse as oil-displacing media. lfsuch fluids, however, are employed informations which are shaley or have shale streaks, there is a tendencyfor the shales to interfere with the effectiveness and stability of theemulsions. This tendency can be reduced through application of thepresent invention by controlling the vapor pressure of the aqueous phaseof the emulsions of microemulsions so that it is substantially equal toor less than the aqueous vapor pressure of the shaley constituents ofthe formation. The manner of control is the same as that described fordrilling fluids earlier in this disclosure. Since the vapor pressure ofdroplets of a liquid become significantly higher than the vapor pressureof the bulk liquid itself it the droplets are small enough, dropletdiameter can be a design consideration in formulating microemulsiondisplacement fluids. Data on mal pressure zone and that exhibits amarked increase in water the effect of droplet diameter is presented byPaul Becher on page 8 of Emulsions: Theory and Practice, ReinholdPublishing Corporation, New York 1957).

C. Fracturing Fluids Many of the hydraulic fracturing fluids used tostimulate oil wells contain water. When such fracturing fluids are usedin the presence of argillaceous, water-sensitive formations, theformations tend to swell and are thereby damaged. This damage can beprevented by using oil-base or water-in-oil invert emulsion fracturingfluids prepared in accordance with this invention. For fracturing fluidsthe amount of vapor pressure depressant added to the aqueous phase ofthe emulsion fluid should be sufficient to reduce the aqueous vaporpressure of the emulsion fluid to a level substantially equal to that ofthe water-sensitive formation. A particularly successful fracturingmethod which is described in U.S. Pat. No. 3,378,074 utilizes a viscousdispersion of water-in-oil as a fracturing fluid. The viscous fluid islubricated down the borehole by means'of an annular ring of water. Sincethe fracturing fluid and the annular ring are subjected to extremeturbulence as the combined stream is forced through perforations andinto the formation to be fractured, it appears that at least temporarilyboth combine to form a water-in-oil emulsion. As a result, it isdesirable to add a vapor pressure depressant to both the internal phaseof the fracturing fluid and the water used to form the annular ring.

D. Vapor Pressures of Penneable Formations Where emulsion fluidsdesigned in accordance with the present invention are to be introducedinto permeable, watersensitive formations, e.g., oil-producingformations, another technique for determining the aqueous vapor pressureof the formation should be considered. This method stems from the factthat such formations normally contain connate water in a fraction of thepore space occupied by reservoir fluids. This connate water is generallyhighly saline, frequently containing salts in concentrations to orexceeding several hundred thousand parts per million. Both the connatewater 7 and the salt ions contained therein freely contact theargillaceous water-sensitive material contained in the formation. Theconnate water will therefore normally be in equilibrium with theargillaceous material so that the aqueous activity of the formationwater will be equal to that of the water-sensitive formation. It willthus frequently be convenient to obtain a sample of this formation waterand determine its aqueous activity in lieu of obtaining and analyzingformation samples. Produced brine serves as a particularly convenientsource of such fluid samples. The aqueous activity of the formationwater can readily be determined by placing a water sample in a closedcontainer and directly measuring the relative humidity of the atmospherein contact with the water.

Ill. General It will be understood, and particularly so with respect tothe claims which follow, that while numerous references are made hereinto the aqueous activity, relative humidity or relative vapor pressure ofmaterials, e.g., earth formations, samples of such formations andwater-containing fluids, in each case the quantitative value referred tois the ratio of the aqueous vapor pressure of the material to the vaporpressure of water at the same temperature. This ratio is proportional tothe aqueous vapor pressure of the material, can be measured rapidly andaccurately and has proved to be a convenient quantitative value forcharacterizing the aqueous vapor pressures of materials employed oracted upon in association with the pared to the absolute aqueous vaporpressure. This is particuarly advantageous where measurements arecarried out on well fluids and formation samples in the laboratory or inthe field at ambient conditions for the purpose of designing emulsionfluids for downhole conditions. That measurements of relative aqueousvapor pressure conducted at atmospheric conditions of temperature andpressure are very good approximations of downhole conditions has beendemonstrated repeatedly by the excellent results achieved when usingfluids designed by these techniques in actual well drilling operations.

What is claimed is:

l. A method of displacing oil from a subsurface, water-sensitiveoil-bearing formation which comprises determining the aqueous vaporpressure of said water-sensitive formation and injecting into theformation through an input well an oil-continuous displacing fluidhaving water dispersed therein which fluid has an aqueous vapor pressureno greater than that of the formation, and recovering oil displacedthereby from the for mation at a point removed from the point ofinjection.

2. A method as defined in claim 1 in which said displacing fluid has anaqueous vapor pressure about equal to the aqueous vapor pressure of thewater-sensitive formation.

3. A method as defined in claim 1 in which the displacing fluid has anaqueous vapor pressure about equal to the aqueous vapor pressure of asample of water from said water-sensitive formation.

4. A method as defined in claim 3 wherein an aqueous vapor pressuredepressant is contained in the water dispersed in said displacing fluid.

5. A method as defined in claim 4 wherein said aqueous vapor pressuredepressant is sodium chloride.

6. A method as defined in claim 4 wherein said aqueous vapor pressuredepressant is calcium chloride.

7. A method as defined in claim 1 in which said displacing fluid is amicroemulsion.

8. A method as defined in claim 7 in which said microemulsion fluid hasan aqueous vapor pressure about equal to the aqueous vapor pressure ofthe water-sensitive formation.

9. A method as defined in claim 7 in which the aqueous vapor pressure ofsaid microemulsion is about equal to the aqueous vapor pressure of asample of water from said watersensitive formation.

10. A method as defined in claim 9 wherein an aqueous vapor pressuredepressant is contained in the aqueous phase of said microemulsionfluid.

11. A method as defined in claim 10 wherein said aqueous vapor pressuredepressant is sodium chloride.

12. A method as defined in claim 10 wherein said aqueous vapor pressuredepressant is calcium chloride.

13. A method as defined in claim 7 in which said microemulsion is amicroemulsion of water in soluble oil.

1! i l i t

2. A method as defined in claim 1 in which said displacing fluid has anaqueous vapor pressure about equal to the aqueous vapor pressure of thewater-sensitive formation.
 3. A method as defined in claim 1 in whichthe displacing fluid has an aqueous vapor pressure about equal to theaqueous vapor pressure of a sample of water from said water-sensitiveformation.
 4. A method as defined in claim 3 wherein an aqueous vaporpressure depressant is contained in the water dispersed in saiddisplacing fluid.
 5. A method as defined in claim 4 wherein said aqueousvapor pressure depressant is sodium chloride.
 6. A method as defined inclaim 4 wherein said aqueous vapor pressure depressant is calciumchloride.
 7. A method as defined in claim 1 in which said displacingfluid is a microemulsion.
 8. A method as defined in claim 7 in whichsaid microemulsion fluid has an aqueous vapor pressure about equal tothe aqueous vapor pressure of the water-sensitive formation.
 9. A methodas defined in claim 7 in which the aqueous vapor pressure of saidmicroemulsion is about equal to the aqueous vapor pressure of a sampleof water from said water-sensitive formation.
 10. A method as defined inclaim 9 wherein an aqueous vapor pressure depressant is contained in theaqueous phase of said microemulsion fluid.
 11. A method as defined inclaim 10 wherein said aqueous vapor pressure depressant is sodiumchloride.
 12. A method as defined in claim 10 wherein said aqueous vaporpressure depressant is calcium chloride.
 13. A method as defined inclaim 7 in which said microemulsion is a microemulsion of water insoluble oil.